Within the field of automotive engineering, it is becoming increasingly common to use turbo-charged engines, i.e. engines in which supercharging takes place with the aid of a compressor driven by an exhaust-gas-driven turbine. At high power a greater 15 charge is used than in a normal engine. In order to avoid knocking and excessively high combustion pressure, the charging pressure is limited and the ignition effected relatively late. This means, in turn, that the opportunity for the burnt gases to expand is small, at the same time as the drop in temperature during expansion does not amount to very much. The temperature of the exhaust gas therefore becomes high. The problems are aggravated by the fact that the share of residual gases in the cylinder is high, with accompanying heating of a new charge and the risk of premature ignition thereof. In order to obtain, as far as possible, a manageable exhaust-gas temperature, it is customary, where the power take-off is high, to use a rich mixture for the engine, with surplus fuel as cooling medium in the cylinder.
The phenomenon of knocking under high load is associated with the fact that a certain part of the charge is self-ignited before the flame front has reached this part of the charge. The result is a very rapid combustion and a pressure wave, or knocking. The final temperature in the cylinder is dependent upon the initial temperature, so that it is desirable for the initial temperature to be as low as possible. One problem in this connection is however that it is difficult to empty the cylinder of hot residual gases, whereby these gases raise the initial temperature and thereby reduce the prospect of pressure increase and power take-off.
It is previously known to use exhaust-gas, recirculation in a turbo engine to render knocking more difficult. Between the conventional exhaust collector and the exhaust-gas turbine, exhaust gases have been drawn off, and, after cooling, recirculated to the engine. A drawback with this solution is that the pressure in the exhaust collector increases when the recirculated gases component increases, whereby the proportion of hot residual gases in the cylinder also increases, so that the temperature in the cylinder prior to compression cannot be lowered to the desired extent.
The object of the invention is to achieve a turbo engine which has improved characteristics, especially under high load. A further object is to achieve this with as simple means as possible.
The object of the invention is achieved by the construction of a combustion engine defined in the introduction and having the distinguishing features defined in Patent claim 1.
As a result of the chosen construction with divided exhaust-gas flow, it is possible, especially under high load, to achieve a more effective emptying of the cylinder, thereby enabling a greater quantity of cooled exhaust gases to be recirculated. The temperature in the cylinder prior to compression can therefore be lowered to a level which has not hitherto been possible. This in combination with the fact that the exhaust gases lower the combustion rate means that the power take-off can be improved. In addition, there is better utilization of fuel by virtue of the fact that fuel does not need to be injected in excess for use as a cooling medium, as has previously been the case.
With the aid of the proposed control valves proposed according to the invention, it is possible, if so desired, to control the exhaust-gas recirculation in different ways in order to obtain the desired engine characteristics. It can be desirable, for example, to open the control valve which allows exhaust-gas recirculation gradually, at the same time as the second control valve is closed gradually. Other variants are also possible, depending upon how the engine is required to respond under, for example, different loads.
It is also conceivable to control the engine valves in such a way that they open and close in different ways under different types of load, possibly in combination with the said control valves being controlled in a certain manner.
The fact that knocking is made more difficult not only yields advantages from a functional aspect, but also reduces the risk of damage to the engine.
Further distinguishing features and advantages of the solution according to the invention can be derived from the description and the other patent claims.
The invention will be described in greater detail below with reference to an illustrative embodiment shown in the drawing.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.